Technologies to produce chemicals and fuels from renewable bio-resources have been the focus of intense development as global reserves of fossil fuels fall and their prices rise. In particular lignocellulosic biomass, containing cellulose, hemicellulose and lignin, is considered an important future resource in the context of a biorefinery. Whilst technologies for the production of ethanol derived from the cellulosic component of lignocellulosic biomass are already in commercial operation, processes making use of the lignin component are still limited. This constitutes a significant drawback in biorefineries where the valorization of all components of the biomass is necessary for economic viability. In addition, despite lignin being a major by-product of the pulp and paper industry, only 2% of the lignin currently produced is used commercially. The valorization of lignin has therefore become an important challenge that has yet to be solved.
Lignin constitutes between 15-30% of the biomass and up to 40% of the energy content of most higher plants. The heterogeneous nature of lignin means that the ratio of monomers and structural units depends highly on the plant source and the extraction process. However the major structural unit is, in almost all lignins, the β-O-4 (β-aryl ether) linkage with lower amounts of β-β (resinol), β-5 (phenylcourmaran), β-1 (1,2-diarylpropane), 5-5 (biphenyl) and 4-O-5 (diaryl ether) linkages. A schematic view of a lignin polymeric structure and illustration of the various types of linkage between aromatic moieties is provided in Scheme 1, below. Sinapyl alcohol derived and coniferyl alcohol derived units may be designated S and G respectively.

The major hurdle to lignin utilisation for the production of biorenewable chemicals is the selective depolymerisation of this recalcitrant material to monomers. It is for this reason that new chemical processes are needed. Several approaches have been suggested. Recently, the selective oxidation of lignin combined with a second chemical treatment has been suggested as a method to bring about the selective depolymerisation of lignin. Rahimi et al (Ref 1), show oxidation of lignin model compounds with molecular oxygen in the presence nitric and hydrochloric acids and 4-acetamido-TEMPO, followed by a second step of cleavage at C—C bonds with hydrogen peroxide under basic conditions. They suggest the possible application to lignins. Nguyen et al (Ref 2) propose a two-step process, also demonstrated only on model compounds, where an oxidation with Bobbitt's salt ([4-AcNH-TEMPO]BF4) is the first step. This can be followed as a second step with a cleavage of C—O bonds using an amine hydrogen donor and formic acid in the presence of an iridium complex and light.
There remains the need to find new approaches to obtaining useful substances from lignin.